The objective is to investigate the three-dimensional structures of thrombin and of neurotoxin by means of X-ray diffraction analysis of crystals of the substances. We have grown crystals of thrombin and have determined the crystallographic system end unit cell dimensions. We intend to solve the molecular structure, investigate the active site by difference Fourier methods, and attempt to obtain some useful information regarding the mechanism of blood clotting, the structure of glycoproteins and add to the knowledge of serine proteinases. We have obtained heavy atom derivatives and computed a 3 A Fourier map of neurotoxin showing the main features of the molecule. We propose to extend the work to 1.7 A, determine the atomic coordinates and refine the structure, which should be useful to students of the neuromuscular junction, as the toxin appears to block specifically and irreversibly the excitation by cholinergic agonists.